Triggered by JDK-8331348, I came to the conclusion that SetupExecute should cd to a temporary directory before executing a command, and then afterwards check that it is empty, to catch any junk files created in the current directory by that command.

Not only will it help keep the git source repo clean, such files can be a signal of a problem with the command, and they are easily missed. Now the build system can get a chance to signal to the user that something unexpected happened, and possibly even abort.